Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 30, 1992
NRC INFORMATION NOTICE 92-33: INCREASED INSTRUMENT RESPONSE TIME WHEN
PRESSURE DAMPENING DEVICES ARE INSTALLED
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to increased response times for pressure sensing
instruments that occur when pressure dampening devices are installed in the
instrument sensing lines. It is expected that recipients will review the
information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems. However, suggestions contained in
this information notice are not NRC requirements; therefore, no specific
action or written response is required.
Description of Circumstances
On September 25, 1991, the GPU Nuclear Corporation, licensee for the Oyster
Creek Nuclear Generating Station, initiated a plant shutdown after
determining that seven of eight isolation condenser line break pressure
sensors did not meet the plant technical specification requirements for
instrument response times. The licensee's trouble shooting determined that
pressure dampening devices in the sensing lines for these differential
pressure sensors had caused an increase in the response times. Furthermore,
the licensee found that the time delay caused by these devices is
significant at both low and high pressures.
Discussion
The pressure dampening devices (snubbers) which utilize sintered stainless
steel elements are generally installed in instrument sensing lines to dampen
pressure oscillations or to protect the instruments from particulate
contamination.
When the sensors failed to meet the required response times during
surveillance tests, the licensee reviewed the pressure sensing system and
found that the snubbers were causing unacceptable time delays. The licensee
later removed the snubbers because they are not needed when using the
upgraded Barton pressure sensors that were installed.
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IN 92-33
April 30, 1992
Page 2 of 2
Snubber time delay will not be detected in response time testing conducted
directly at the sensing instrument (see Figure 1.) The effect of a snubber
will differ from sensor to sensor because of differences in the volumetric
displacement of fluid within the pressure sensing mechanisms. System
response time can also be degraded by the accumulation of foreign material
in sensing line snubbers.
The NRC's Office of Nuclear Regulatory Research is conducting a generic
study on the performance of pressure instrumentation at nuclear power
plants. The staff plans to publish the results in NUREG/CR 5851, "Long Term
Performance and Aging Characteristics of Nuclear Plant Pressure
Transmitters." The staff has completed a part of this effort, and the
results of tests conducted on pressure sensor response time testing were
published in an ISA (Instrument Society of America) transaction 91-720,
"Response Time Testing of Pressure Transmitters in Nuclear Power Plants."
This publication addresses various causes for delays in sensor response and
documents that significant time delays can occur.
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Thomas Koshy, NRR
(301) 504-1176
Iqbal Ahmed, NRR
(301) 504-3252
Attachments:
1. Figure 1. Test Configuration that Excludes the
Effect of Snubbers
2. List of Recently Issued NRC Information Notices
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